The changing natural world at our doorsteps | Illustration and text by Patterson Clark
January 25, 2011
Volunteers help control invasive ivy
In winter, ivy-covered trees stand out as stark green pillars of exotic invasion
John Bowman cuts an ivy vine from the base of a tree not far from his home in Washington's North Portal Estates. Bowman is a volunteer with Friends of Rock Creek's Environment, which helps Rock Creek Park with stream cleanup and invasive plant removal.
In January, tree-climbing English and Irish ivy bears black-purple fruit that lures hungry birds.
That's good for the ivy but sometimes bad for the bird. Glycosides in the fruit can cause some birds to vomit, which further spreads the seed of the weed. Starlings and house sparrows — two other British imports run amok — aren't sickened by the berries, so their droppings may carry the seeds even farther.
Noxiously invasive ivy starts out as a ground cover, too often planted as an ornamental. It can relentlessly swarm over and replace native herbaceous plants. Thick mats of ivy will also thwart the growth of tree seedlings.
Worse trouble happens when the vine climbs into trees: The plant becomes a beast, its vine swelling into a woody stem as fat as your ankles. The stem sprouts branches bearing a spearhead-shaped leaf, quite different in shape from that of the ground-bound vine. Flowers appear, fruits ripen, the tree suffers.
"Ivy can kill trees," says Ken Ferebee of Rock Creek Park. It can envelop branches and twigs, suppressing tree growth and "causing a decline in the tree that can last for many years," says Ferebee. The weed adds extra weight to its host tree, and its dense growth catches the wind like a sail, increasing the likelihood that a tree will be blown over.
Because English ivy and Irish ivy have no natural predators, competition or diseases, the task of controlling the weed falls to people.
Armed with pruners, loppers and saws, park volunteers liberate trees from ivy by cutting the weed off at its knees, removing ivy stems from a zone around the base each tree. Park specialist Joe Kish cautions his volunteer crew not to peel off the tree's bark while removing ivy. "That can introduce diseases into the tree," he explains, "Instead of helping the tree, you've damaged it."
After the cut, "ivy will stay green for a while," says Ferebee, "but it will eventually turn brown and fall off."
SOURCES: National Park Service, Friends of Rock Creek's Environment, noivyleague.com
NOTE: An earlier version of this article identified the plant as English Ivy; however, much of Washington's ivy infestation is of a closely related species, Irish Ivy.
Before and after: Ivy-infested trees in a corner of Whitehaven Park, left, and a year after treatment, right.
Jekyll and Hyde
Ivy as a ground cover, bottom, transforms into a tree-killing monster, top, when it climbs.